Optical focus sensor

Abstract

Recovery from a record of data recorded in an information track, comprising depressed areas of a given width, a given depth, and variable length, alternating along the length of the track with relatively non-depressed areas, is effected by an optical playback system. The playback system employs means for focusing light from a laser source on the information track as a light spot having a diameter which is diffraction limited. The focusing system includes an independent light source (e.g., laser) which impinges on the surface of the record as a circular spot which illuminates a plurality of elongated tracks. The circular spot is imaged by a lens system on to a detector plane which is positioned at a plane which is conjugate to the record surface. The focus error signal which is used to adjust the position of the objective lens with respect to the record surface is representative of a measure of the diameter of the circular spot imaged on the detector plane, and, thus, is a measure of the diameter of the circular spot at the record surface.

What is claimed is: 
     
       1. A focus control apparatus for use with a disc-shaped record medium having information stored in a spiral groove formed on a flat, reflective surface thereof, the apparatus comprising: a first light source emitting a beam of light of a first given wavelength;   a first light path optically coupling said first light source and said reflective surface of said record medium, said first light path including means for focusing said light beam emitted by said first light source to a diffraction limited spot on said surface of said record medium;   first light detection means having a photosensitive surface, said light detection means being used to detect said information stored in said spiral groove;   a second light path optically coupling said focused light spot on said surface of said record medium to said photosensitive surface of said first light detection means;   a second light source emitting a beam of light of a second given wavelength;   a third light path optically coupling said second light source and said reflective surface of said record medium, said third light path including said means for focusing, said light beam emitted by said second light source being projected by said means for focusing to impinge on said reflective surface as a circular region having a diameter sufficiently large to span a plurality of groove convolutions of said spiral groove;   second light detection means having four photosensitive regions arranged to measure the diameter of an illuminated spot incident on said second light detection means;   a fourth light path coupling said circular region on said reflective surface to said second light detection means and including means for imaging a conjugate image of said circular region onto said second light detection means; and   means, responsive to said second light detection means, for generating a signal representative of the diameter of said conjugate image; and   means, responsive to said signal generated by said generating means, for varying the distance between said focusing lens and said reflective surface of said record medium to maintain said light beam emitted by said first source in-focus on said reflective surface.   
     
     
       2. The apparatus according to claim 1 wherein said first given wavelength is 633 nm and wherein said second given wavelength is 820 nm. 
     
     
       3. The apparatus according to claim 1 wherein said four photosensitive regions include: a first pair of photosensitive regions positioned on said second light detection means to be illuminated by said illuminated spot when an in-focus condition exists; and   third and fourth photosensitive regions, said third photosensitive region being positioned adjacent a first one of said pair of photosensitive regions and partially illuminated by said circular area when said in-focus condition exists and said fourth photosensitive region being positioned adjacent a second one of said pair of photosensitive regions and partially illuminated by said circular area when said in-focus condition exists.   
     
     
       4. The apparatus according to claim 3 further comprising a stop located in said third light path.